SALT LAKE CITY — A Utah military base that carries out tests to protect troops against biological attacks was locked down for nearly 12 hours overnight because a small amount of a nerve agent was unaccounted for.

It reopened early Thursday, officials said, after the missing vial was located.

The military said in a statement Thursday the amount missing was less than one fourth of a teaspoon of VX nerve agent, which affects the body's ability to carry messages through the nerves.

Philip Seymour Hoffman withdrew a total of $1,200 from an ATM at a supermarket near his New York City apartment the night before he was found lifeless in his bathroom with a syringe still in his left arm, sources told NBC News.

Dugway Proving Grounds spokeswoman Paula Thomas said the base, located about 85 miles southwest of Salt Lake City, had first reopened to incoming personnel, and people inside were allowed to leave shortly thereafter.

She said there were no injuries resulting from the cause of the lockdown. About 1,200 to 1,400 people — a mix of military personnel and contractors and civilian workers — were inside the base when the lockdown occurred, Thomas said.